The present invention relates to a wireless communication system, a wireless communication apparatus and a wireless communication method, and a computer program. The wireless communication system communicates with a plurality of radio stations with each other, such as a wireless LAN (Local Area Network) or the like. The present invention particularly relates to a wireless communication system, a wireless communication apparatus and a wireless communication method, and a computer program in which a wireless network is operated by direct communication (random access) between communication stations.
More particularly, the present invention relates to a wireless communication system, a wireless communication apparatus and a wireless communication method, and a computer program in which communication stations perform network operation on an autonomous distributed basis without having a relation of a control station and controlled stations. Particularly, the present invention relates to a wireless communication system, a wireless communication apparatus and a wireless communication method, and a computer program in which communication stations operating on an autonomous basis form an autonomous distributed wireless network without interfering with each other while maintaining synchronization with each other.
Standards for wireless networks include for example IEEE (The Institute of Electrical and Electronics Engineers) 802.11(see Non-Patent Document 1, for example), HiperLAN/2(see Non-Patent Document 2 and Non-Patent Document 3, for example), IEEE802.15.3, and Bluetooth communication. As to the IEEE802.11 standard, there are extension standards such as IEEE802.11a (see Non-Patent Document 4, for example), IEEE802.11b, IEEE802.11g, and the like for different wireless communication systems and different frequency bands being used, for example.
Generally, in order to form a local area network using radio technology, a method is used in which one apparatus serving as a control station referred to as an “access point” or a “coordinator” is provided within an area and a network is formed under centralized control of the control station.
In wireless networks having an access point disposed therein, an access control method based on band reservation is widely used in which when information is to be transmitted from a communication apparatus, first a band required to transmit the information is reserved at the access point to use a transmission path so as to prevent collision with information transmitted by another communication apparatus. Synchronous wireless communication in which communication apparatuses within a wireless network establish synchronization with each other is performed by disposing the access point.
However, when asynchronous communication is performed between communication apparatuses on a sender side and a receiver side in a wireless communication system having an access point, the wireless communication always needs to be performed via the access point, thus halving efficiency of use of a transmission path.
On the other hand, as another method for forming a wireless network, “ad hoc communication” has been devised in which terminals perform wireless communication with each other directly on an asynchronous basis. For a small-scale wireless network formed by a relatively small number of clients located in the vicinity of each other, ad hoc communication is considered suitable. The ad hoc communication enables arbitrary terminals to perform direct wireless communication, that is, random wireless communication with each other without using a specific access point.
An ad hoc type wireless communication system, which has no central control station, is suitable for forming a home network including household electric appliances, for example. An ad hoc network has for example a feature of resisting failure because routing is automatically changed even when one appliance fails or is turned off, and a feature of being able to transmit data to a relatively long distance while maintaining a high data rate by hopping a packet a plurality of times between mobile stations. Various developed examples of the ad hoc system are known (see Non-Patent Document 5, for example).
For example, IEEE802.11 networking in an IEEE802.11 wireless LAN system is based on a concept of a BSS (Basic Service Set). There are two kinds of BSS: a BSS defined by an “infrastructure mode” in which a master such as an access point (control station) or the like is present, and an IBSS (Independent BSS) defined by an “ad hoc mode” in which the BSS is formed by only a plurality of MTs (Mobile Terminals). In the latter ad hoc mode, the terminals perform peer-to-peer operation on an autonomous distributed basis without a control station being disposed. When a beacon transmission time comes, each terminal counts a random period. When no beacon is received from the other terminals before the period is ended, the terminal transmits a beacon.
Details of conventional wireless networking will be described in the following by taking IEEE802.11 as an example.
Infrastructure Mode
A BSS in the infrastructure mode requires an access point for performing coordination within a wireless communication system. Specifically, the access point groups stations within reach of radio waves in the vicinity of the own station as a BSS to form a so-called “cell” in a cellular system. A mobile station present in the vicinity of the access point is accommodated by the access point, and enters the network as a BSS member.
The access point transmits a control signal referred to as a beacon at appropriate time intervals. A mobile station that can receive the beacon recognizes presence of the access point in the vicinity, and then establishes connection with the access point.
The mobile station in the vicinity of the access point can receive the beacon, decode a TBTT field within the beacon, and recognize a next beacon transmission time. Therefore, in some cases (cases where reception is not required), the mobile station may turn off a receiver to be in a sleep state (to be described later) until a next TBTT or a TBTT after one or more TBTTs.
In the infrastructure mode, only the access point transmits a beacon in a predetermined frame cycle. On the other hand, mobile stations in the vicinity enter the network by receiving the beacon from the access point, and the mobile stations do not transmit a beacon. Incidentally, since the present invention mainly focuses on operating a network without intervention of a master control station such as an access point or the like and is thus not directly related to the infrastructure mode, further description of the infrastructure mode will not be given.
Ad Hoc Mode
IEEE802.11 operation in the other ad hoc mode will be described with reference to FIG. 27.
A plurality of mobile stations in an IBSS in the ad hoc mode negotiates with each other, and then autonomously defines the IBSS. After the IBSS is defined, the mobile station group sets TBTTs at regular intervals after negotiation. When each mobile station recognizes that a TBTT arrives by referring to a clock within the own station, and when the mobile station recognizes that no beacon is transmitted yet after a delay of a random time, the mobile station transmits a beacon.
FIG. 27 shows an example in which two mobile stations form an IBSS. In this case, either one of the mobile stations belonging to the IBSS transmits a beacon each time a TBTT arrives. There is a case of collision between beacons transmitted from the respective mobile stations.
The mobile stations in the IBSS may also go into a sleep state by turning off a transmitter-receiver (to be described later) as required.
Procedure for Transmission and Reception in IEEE802.11
It is generally known that a hidden terminal problem occurs in a wireless LAN network of an ad hoc environment. When communication is performed between specific communication stations, a hidden terminal is a communication station that can be heard by one communication station with which the communication is performed but cannot be heard by another communication station. Since hidden terminals cannot negotiate with each other, there can be a collision in transmission operation.
As a method for solving the hidden terminal problem, CSMA/CA based on an RTS/CTS procedure is known. IEEE802.11 employs this method.
CSMA (Carrier Sense Multiple Access with Collision Avoidance) is a connection system for performing multiple access on the basis of carrier detection. In wireless communication, it is difficult for a communication station to receive an information signal transmitted by the communication station itself. Therefore, by the CSMA/CA (Collision Avoidance) system rather than CSMA/CD (Collision Detection), the communication station checks that no information is transmitted from other communication apparatuses, and then starts transmitting information of the communication station, thus avoiding collision.
In the RTS/CTS system, a communication station as a data sender source transmits a transmission request packet RTS (Request To Send), and then starts transmitting data in response to an acknowledgement packet CTS (Clear To Send) from a communication station as a data transmission destination. Receiving at least one of the RTS and the CTS, a hidden terminal sets a transmission stop period of the own station for expected period to transmit data based on the RTS/CTS procedure to be performed, whereby collision can be avoided. A terminal hidden from a sender station receives a CTS and then sets a transmission stop period to avoid collision between data packets. A terminal hidden from a receiver station receives an RTS and then sets a transmission stop period to avoid collision between ACKs.
Procedure for Transmitting and Receiving Signal in Sleep State
In IEEE802.11 networking, a mobile station in an IBSS at the time of the ad hoc mode may also go into a sleep state by turning off a transmitter-receiver as required.
When the sleep mode is applied in the IBSS of IEEE802.11, a certain period of time from a TBTT is defined as an ATIM (Announcement Traffic Indication Message) window. For the time period of the ATIM window, all mobile stations belonging to the IBSS operate a receiver. During this time period, a mobile station operating in the sleep mode is basically capable of reception.
When each mobile station has information for a destination, the own station transmits an ATIM packet to the communication destination during the time period of the ATIM window, thereby notifying the receiver side that the own station has the information to be transmitted. The mobile station receiving the ATIM packet operates a receiver until reception from the station that transmitted the ATIM packet is ended.
FIG. 28 shows an example in which three mobile stations STA1, STA2, and STA3 are present in an IBSS. When a TBTT arrives, the mobile stations STA1, STA2, and STA3 each operate a back-off timer while monitoring a state of a medium for a random time. In the example shown in FIG. 28, the timer of the mobile station STA1 expires the earliest, and the mobile station STAL transmits a beacon. Since the mobile station STAL transmits the beacon, the mobile station STA2 and the mobile station STA3 receiving the beacon do not transmit a beacon.
In the example shown in FIG. 28, the mobile station STA1 has information for the mobile station STA2, and the mobile station STA2 has information for the mobile station STA3. In this case, after the beacon is transmitted and received, the mobile stations STA1 and STA2 each operate the back-off timer while monitoring a state of the medium for a random time again. In the example shown in FIG. 28, the timer of the mobile station STA2 expires earlier, and the mobile station STA2 transmits an ATIM message to the mobile station STA3. Receiving the ATIM message, the mobile station STA3 gives a feedback indicating the reception of the ATIM message to the mobile station STA2 by transmitting an ACK (Acknowledge) packet.
After the transmission of the ACK is completed, the mobile station STA1 further operates the back-off timer while monitoring a state of the medium for a random time. When the timer expires, the mobile station STA1 transmits an ATIM packet to the mobile station STA2. The mobile station STA2 gives a feedback to the mobile station STA1 by returning an ACK packet indicating the reception of the ATIM packet.
During an interval after the ATIM packets and the ACK packets are exchanged within an ATIM window, the mobile station STA3 operates a receiver to receive information from the mobile station STA2, and similarly the mobile station STA2 operates a receiver to receive information from the mobile station STA1.
After the end of the ATIM window, the mobile station STA1 and the mobile station STA2 having the information to be transmitted each operate the back-off timer while monitoring a state of the medium for a random time. In the example shown in FIG. 28, the timer of the mobile station STA2 expires earlier, and thus the mobile station STA2 first transmits the information to the mobile station STA3. After the transmission of the information is completed, the mobile station STA1 operates the back-off timer while monitoring a state of the medium for a random time again. When the timer expires, the mobile station STA1 transmits a packet to the mobile station STA2.
In the above-described procedure, a communication station receiving no ATIM packet within the ATIM window and a communication station having no information to be transmitted keep a transmitter-receiver turned off until a next TBTT to thereby reduce power consumption.
[Non-Patent Document 1]
International Standard ISO/IEC 8802-11: 1999 (E) ANSI/IEEE Std 802.11, 1999 Edition, Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications
[Non-Patent Document 2]
ETSI Standard ETSI TS 101 761-1 V1. 3. 1 Broadband Radio Access Networks (BRAN); HIPERLAN Type 2; Data Link Control (DLC) Layer; Part 1: Basic Data Transport Functions
[Non-Patent Document 3]
ETSI TS 101 761-2 V1. 3. 1 Broadband Radio Access Networks (BRAN) HIPERLAN Type 2; Data Link Control (DLC) Layer; Part 2: Radio Link Control (RLC) sublayer
[Non-Patent Document 4]
Supplement to IEEE Standard for Information technology—Telecommunications and information exchange between systems—Local and metropolitan area networks—Specific requirements—Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: High-speed Physical Layer in the 5 GHZ Band
[Non-Patent Document 5]
C. K. Tho, “Ad Hoc Mobile Wireless Network” (published by Prentice Hall PTR)
As described above, in a wireless communication system in which communication stations perform network operation on an autonomous distributed basis without having a relation of a control station and controlled stations, each communication station makes presence of the communication station itself known to the other communication stations in the neighborhood (that is, within a range of communication) by sending beacon information periodically on a channel, and notifies a network configuration. Since the communication station transmits a beacon at a start of a transmission frame cycle, the transmission frame cycle is defined by a beacon interval. Also, each communication station can detect a beacon signal transmitted from neighboring stations by performing scan operation on the channel for a period corresponding to the transmission frame cycle, and be informed of the network configuration (or enter the network) by decoding information described in the beacon.
In such a wireless communication system, each communication station needs to periodically send beacon information and manage beacon information as described above while maintaining time synchronization with the neighboring stations. Also, in an access method based on time synchronization in which the communication station reserves a band or sets a priority use period within a frame cycle, for example, it is very important for communication stations to maintain time synchronization with each other.
In many of conventional wireless communication systems requiring time synchronization, each communication station establishes time synchronization with a neighboring station through intervention of a control station, and thereby time synchronization can be established between communication stations present in the same network.
On the other hand, since a wireless communication system in which communication stations perform network operation on an autonomous distributed basis does not have a relation of a control station and controlled stations, a conventional method of establishing time synchronization cannot be applied to such a wireless communication system. For example, in the wireless communication system of the autonomous distributed type, a method is conceivable which temporarily defines a master-slave relation between communication stations and thus creates a communication station temporarily serving as a control station within the same network, whereby a conventional method is applied to establish time synchronization. However, processes for determining the master and the slave and for resetting the master when the master disappears from the network are complicated.
Since each communication station in ad hoc communication has a greater process load than in infrastructure communication performed via an access point or the like, further increase in the process load is not desirable. Thus, the autonomous distributed type communication system requires techniques that enable time synchronization to be established between communication stations by a relatively simple process.